Conventionally, lithium ion batteries and the like are commonly configured as a laminate battery which is composed of an electrode plate group and an electrolyte encased in an exterior case made of laminate sheets. Lithium ion batteries which have metal battery cases are generally provided with a safety vent that opens part of the battery case to the outside so as to release internal gas smoothly to the outside when the temperature or internal pressure of the battery exceeds a predetermined level, in order to prevent rupture of the battery which results from a battery temperature rise and a subsequent rapid pressure build-up. There are usually two types of safety vents: Heat shut-off type and rupture membrane type. Most batteries adopt one of them, and some use both of them.
Some cylindrical batteries that use the above metal battery case include a built-in resealable safety vent. For example, a known resealable safety vent 63 shown in FIG. 17A includes an exhaust hole 61 provided in a sealing plate 60 and a rubber valve element 62 that seals the exhaust hole 61 by making pressure contact with the periphery of the hole 61. Another known resealable safety vent 66 shown in FIG. 17B includes an exhaust hole 61 similarly to the above, a rubber-lined valve element 64, and a spring 65 that applies a force in a direction in which the valve element 64 is pressed against the periphery of the exhaust hole 61 to seal the hole 61. Such resealable safety vents 63 and 66 cannot be applied to laminate batteries. Application of such vents to laminate batteries is unthinkable because of large functional difference, and of the inevitable large difference in configuration, between the laminate sheet exterior case of the laminate battery and the metal battery case.
In laminate batteries, usually, the laminate sheets that form the exterior case are made of polypropylene (hereinafter, abbreviated as “PP”) and bonded together by heat sealing. Since the melting temperature of PP is approximately 150° C., the laminate sheet melts and releases internal gas to the outside to lower the pressure when the battery temperature exceeds 150° C., thereby preventing a rupture accident. A safety vent configuration commonly adopted in laminate batteries uses plastic that has a lower melting point than PP and is inexpensive as much as possible, for example polyethylene (hereinafter, abbreviated as “PE”); the plastic is interposed in between the bonded part, so that this portion will melt first to open the vent when the battery heats up (see, for example, Patent Document 1).
Another known safety vent configuration for laminate batteries provides a portion where the peeling strength is reduced by making the width of the heat-sealed part narrower, so that this portion will function as a safety vent (see, for example, Patent Document 2).
Yet another known safety vent configuration for laminate batteries provides a portion in the sealed part where the pressure resistance (peeling strength) is reduced by making smaller the thickness of part of the adhesive layer or heat-sealed plastic layer of the sealed part, or by implanting a different material film or a hollow component, so that this portion will function as a safety vent (see, for example, Patent Document 3).
[Patent Document 1] Patent Laid-Open Publication No. 2001-93489
[Patent Document 2] Patent No. 3554155
[Patent Document 3] Patent Laid-Open Publication No. 11-86823